Cyclotron masers, commonly referred to as gyrotrons, are used to produce high power, narrow bandwidth, modally pure microwaves. Conventional cavity structures used in gyrotrons limit the operating frequency range of the gyrotron to a single frequency, or a very narrow frequency range. Gyrotrons with operating frequencies of 35 gigahertz (GHz) and 94 Ghz are common, since these frequencies produce low attenuation in the atmosphere. Producing two or more widely separated microwave frequencies normally requires using two or more independent gyrotrons, which greatly increases system costs due to the need for redundant components including electron guns, power supplies and magnets. Some reduction in cost is achieved by the use of two separate cavities, which is offset by an increased number of man hours needed to replace the various cavities.